I. Field of the Invention
This invention relates to delay compositions used in detonators for explosives (sometimes referred to as blasting caps) and other devices (e.g. inline detonation delay devices), and to detonation delay elements and devices containing such compositions. More particularly, the invention relates to delay compositions having slow-burning (long delay) times for use with both non-electric and electric detonators, inline delay devices, and the like.
II. Background Art
Delay compositions are materials that burn away rapidly, but not instantly, when ignited, thus create a timing delay, in the nature of a fuse, when shaped and compacted in the form of an elongated body or column and ignited at one end. Such compositions may therefore be used to create a delay between the instant at which a detonator or similar device receives a firing signal (which commences ignition of the column of delay composition), and the instant at which an associated explosive charge is set off (by heat when the combustion reaches the remote end of the burning column), or a further firing signal is generated.
Delay detonators and similar delay devices, both non-electric and electric, are widely employed in mining, quarrying and other blasting operations in order to permit sequential initiation of explosive charges distributed in a predetermined pattern of bore holes or shot holes. The provision of a delay between sequential initiation of adjacent bore or shot holes is effective in controlling the fragmentation and throw of the rock being blasted and, in addition, provides a reduction in ground vibration and in air blast noise.
Modern commercial delay detonators, whether non-electric or electric, normally comprise a metallic shell, closed at one end, which contains in sequence from the closed end: a base charge of a detonating high explosive, such as for example pentaerythritoltetranitrate (PETN), and an adjacent primer charge of a heat-sensitive detonable material, such as for example lead azide. Adjacent to the heat-sensitive material is a consolidated, e.g. compressed, column of delay composition of sufficient length and quantity to provide a desired delay time as described above. The column of the delay composition is normally confined within a hollow tubular confinement element made of metal. The confinement element and delay composition contained therein, together with sealers and primer charges, if any, form a delay element that is normally fabricated separately and assembled into a detonator or the like as a single item. Next to the delay element is an ignition (starter) charge adapted to be ignited by an electrically heated bridge wire or, alternatively, by the heat and flame of a low energy detonating cord or shock wave conductor retained in the open end of the metallic shell. Such a delay detonator may serve as an in-line delay as when coupled at both ends to a detonating cord or shock wave conductor. However, a delay device need not also be capable of serving as a detonator in order, for example, to initiate a shock wave conductor. An ignition charge in close proximity to the end of the shock wave conductor, instead of a base charge of detonating high explosive, will suffice.
The containment of the delay composition within a confinement element facilitates the handling of the composition and its introduction into a detonator or the like. The metal also protects other components (e.g. the outer shell of a detonator) from the heat and by-products of combustion as the delay composition is consumed and, for reasons of economy, minimizes the amount of the delay composition that is required. In the past, lead has often been used as the metal for the confinement elements. Lead is soft and malleable and can be loaded with a burning core, drawn to a desired diameter and cut to required lengths (different lengths produce different delay times). Lead also has a low thermal conductivity and heat capacity, and therefore diverts only a minimum amount of heat from the composition as it burns, thus reducing the risk that the combustion may be quenched or extinguished prior to complete consumption of the delay composition.
A trend has recently developed of replacing confinement elements made of lead with elements made of rigid metals, such as zinc, aluminum, steel or brass. Zinc is currently the preferred metal of choice for this purpose. The term “rigid metal” refers to those metals that, when used to form confinement elements, are not easily drawn to a desired diameter or shaped using the equipment currently available for lead. With such metals, the confinement elements are first cast to the desired diameter and length, and then the delay composition is loaded into the interior of the element and compressed. This change to rigid metal confinement elements has come about in part because the use of lead is receiving criticism from some quarters for being environmentally hazardous, even though the quantity of lead is small. Moreover, the use of rigid metal confinement elements can facilitate fabrication of delay units and their integration into detonators and delay devices, etc. However, zinc and other suitable rigid metals have higher thermal conductivities and heat capacities than lead, and thus extract more heat from the delay composition as it burns. This can increase the failure rate of detonators and delay devices because there may be insufficient heat remaining in the delay composition to maintain the combustion temperature until complete consumption of the composition has taken place, especially when such devices are used in low temperature environments. Particularly at risk of failure are delay units intended to provide long delays, e.g. more than one second, often used in underground applications.
A large number of delay compositions are known in the art. These generally comprise mixtures of fuels and oxidizers of various kinds. Many are substantially gasless compositions, which are generally preferred; that is, they burn without evolving large amounts of gaseous by-products which could interfere with the functioning of a delay detonator or other device. In addition to an essential gasless requirement, delay compositions are also required to be safe to handle, from both an explosive and health viewpoint, they must be resistant to moisture and not deteriorate over long periods of storage and hence change in burning characteristics, they must operate reliably over a wide range of temperatures, and they must be adaptable of use in a wide range of delay units within the limitations of space available inside a standard detonator shell or similar device. The numerous delay composition of the prior art have met with varying degrees of success in use and application.
One such prior class of delay compositions intended for use in confinement elements made of lead is that described in U.S. Pat. No. 4,419,154 to Davitt et al. (assigned to CXA Ltd/CXA LTEE) which issued on Dec. 6, 1983. This patent discloses a composition comprising silicon and barium sulfate and optionally including a proportion of particulate red lead (lead tetroxide, Pb3O4) in the amount of 25 to 75% by weight of the composition. The compositions of Davitt which include red lead can be used in confinement elements made of lead to produce intermediate to long timing delays. However, in order to achieve the long timing delays with red read, which is recognized as a strong oxidant, the Davitt compositions have to be prepared with coarse silicon. Such slow burning compositions are difficult to ignite due to the use of such coarse silicon that goes against traditional pyrotechnic principles as taught by Professor Conkling, who stated, in Chemistry of Pyrotechnics, John A. Conkling, Marcel Dekker Inc., 1985, pp 88-89:                “Homogeneity, and pyrotechnic performance, will increase as the particle size of the various components is decreased. The finer the particle size, the more reactive a particular composition should be, with all other factors held constant.”        
Furthermore the slow burning compositions of Davitt et al. with red lead were prepared with a very small ratio of the fuel component (i.e. silicon) which was significantly below the stoichiometric ratio, with the consequence of reducing the energy output of the combustion process. Such formulations would not be robust in various conditions, such as when used in rigid elements as herein described where the thermal conductivity of such confinement materials is significantly higher than lead. Furthermore, when Davitt et al. attempted to use finer silicon with red lead, which would have had the consequence of improving the pyrotechnic performance, significantly faster timing results were obtained (Column 8 of the patent). The only slow burning compositions of Davitt et al. that can be prepared with fine silicon are those without red lead. Thus, according to Davitt et al., compositions with red lead are not ideal for producing long timing delays. For long delay periods, there is therefore a need to find alternative delay compositions.
U.S. Pat. No. 5,147,476 to Beck et al. (assigned to Imperial Chemical Industries PLC), which issued on Sep. 15, 1992, addresses the problem of increasing the robustness of combustion of delay compositions intended for use in rigid metal confining elements to reduce the likelihood of quenching of the combustion. The concept of Beck et al. was to facilitate the combustion of a mixture of silicon and barium sulfate (or other oxidant) by adding small amounts of dispersed metal compounds to serve as reaction-facilitating fluxes (i.e. materials that lower the fusion temperature of the composition, but are otherwise inert). The illustrated metal compounds are salts of alkali metals, oxides of antimony and oxides of vanadium. Beck et al. found that for reliable burning of such a composition, the heat sink effect of the confinement metal element should not be such as to risk quenching of the exothermic reaction (i.e. burning) of the delay composition. However, the delay compositions of the kind disclosed by Beck et al. do not work as well as might be desired, particularly when used for producing long delays. Moreover, the oxides used in these compositions as fluxes are expensive. Beck et al. suggested that additions of red lead oxide or other reactive ingredients that cause a faster rate of burning may be incorporated into the composition, but noted that large loadings of such reactive ingredients may obviate the facilitating role of the flux. Beck et al. therefore recommended that the compositions omit such additional reactive ingredients.
There is therefore a continuing need for a delay composition that can be used reliably when confined in rigid metal confinement elements and yet may be used to produce long delays without unacceptably increasing the lengths of delay units.